1. Field of the Invention
The present invention relates to a semiconductor memory device having an SRAM (static random access memory) and a DRAM (dynamic random access memory) that are provided on a single chip.
2. Description of the Related Art
For example, in conventional portable telephones, SRAMs were often employed as the memory device. Recently, however, there is a demand for portable telephones that are equipped with large-capacity memory devices, such as DRAMs, in order to promote the interfacing between the portable telephones and the Internet.
Generally, in a DRAM memory cell, a read operation must be followed by a subsequent write operation. Charge stored on the capacitor of the memory cell does not remain on the capacitor indefinitely. Due to a variety of leakage paths, the charge can eventually leak off the capacitor, causing the memory cell to loose its information. To alleviate this problem, each memory cell in the DRAM must be periodically read, sensed, and re-written to a full level. This refresh requirement distinguishes the DRAM from the SRAM.
If a portable telephone is equipped with the DRAM, instead of the SRAM, the power consumption in its standby state will be increased since the DRAM requires periodical refresh operations to retain the stored information. Although the recent demand for large-capacity memory devices is met by the DRAM-equipped portable telephone, the time that allows the portable telephone to continuously work in the standby state will be considerably reduced.
The SRAM does not require the refresh operations to retain the stored information and hardly consumes the power during its standby state. However, the density of the storage elements in the SRAM is low compared to the density of the storage elements in the DRAM. If a portable telephone is equipped with a large quantity of the SRAM devices that meets the memory capacity needed by the recent demand, the cost will be significantly increased.